Organic light-emitting diodes and methods for assembly and enhanced charge injection

ABSTRACT

New organic light-emitting diodes and related electroluminescent devices and methods for fabrication, using siloxane self-assembly techniques.

This application is a continuation-in-part of and claims prioritybenefit from application Ser. No. 09/187,891, filed on Nov. 6, 1998, nowU.S. Pat. No. 6,399,221 B1, which is a continuation-in-part ofapplication Ser. No. 08/673,600, filed on Jun. 25, 1996, and issued asU.S. Pat. No. 5,834,100, each of which are incorporated herein byreference in their entirety.

The United States Government has certain rights to this inventionpursuant to Grant Nos. N0014-95-1-1319 and DMR-00769097 from the Officeof Naval Research and National Science Foundation, respectively, toNorthwestern University.

BACKGROUND OF THE INVENTION

This invention relates generally to organic electroluminescent deviceswith organic films between anodic and cathodic electrodes, and moreparticularly to such devices and methods for their assembly using thecondensation of various silicon moieties.

Organic electroluminescent devices have been known, in various degreesof sophistication, since the early 1970's. Throughout their developmentand consistent with their function and mode of operation, they can bedescribed generally by way of their physical construction. Such devicesare characterized generally by two electrodes which are separated by aseries of layered organic films that emit light when an electricpotential is applied across the two electrodes. A typical device canconsist, in sequence, of an anode, an organic hole injection layer, anorganic hole transport layer, an organic electron transport layer, and acathode. Holes are generated at a transparent electrode, such as oneconstructed of indium-tin-oxide, and transported through ahole-injecting or hole-transporting layer to an interface with anelectron-transporting or electron-injecting layer which transportselectrons from a metal electrode. An emissive layer can also beincorporated at the interface between the hole-transporting layer andthe electron-transporting layer to improve emission efficiency and tomodify the color of the emitted light.

Significant progress has been made in the design and construction ofpolymer- and molecule-based electroluminescent devices, forlight-emitting diodes, displays and the like. Other structures have beenexplored and include the designated “DH” structure which does notinclude the hole injection layer, the “SH-A” structure which does notinclude the hole injection layer or the electron transport layer, andthe “SH-B” structure which does not include the hole injection layer orthe hole transport layer. See, U.S. Pat. No. 5,457,357 and in particularcol. 1 thereof, which is incorporated herein by reference in itsentirety.

The search for an efficient, effective electroluminescent device and/ormethod for its production has been an ongoing concern. Severalapproaches have been used with certain success. However, the prior arthas associated with it a number of significant problems anddeficiencies. Most are related to the devices and the methods by whichthey are constructed, and result from the polymeric and/or molecularcomponents and assembly techniques used therewith.

The fabrication of polymer-based electroluminescent devices employs spincoating techniques to apply the layers used for the device. Thisapproach is limited by the inherently poor control of the layerthickness in polymer spin coating, diffusion between the layers,pinholes in the layers, and inability to produce thin layers which leadsto poor light collection efficiency and the necessity of high D.C.driving voltages. The types of useful polymers, typicallypoly(phenylenevinylenes), are greatly limited and most areenvironmentally unstable over prolonged use periods.

The molecule-based approach uses vapor deposition techniques to put downthin films of volatile molecules. It offers the potential of a widechoice of possible building blocks, for tailoring emissive and othercharacteristics, and reasonably precise layer thickness control.Impressive advances have recently been achieved in molecular buildingblocks—especially in electron transporters and emitters, layer structuredesign (three versus two layers), and light collection/transmissionstructures (microcavities).

Nevertheless, further advances must be made before these devices areoptimum. Component layers which are thinner than achievable by organicvapor deposition techniques would allow lower DC driving voltages andbetter light transmission collection characteristics. Many of thedesirable component molecules are nonvolatile or poorly volatile, withthe latter requiring expensive, high vacuum or MBE growth equipment.Such line-of-site growth techniques also have limitation in terms ofconformal coverage. Furthermore, many of the desirable molecularcomponents do not form smooth, pinhole-free, transparent films underthese conditions nor do they form epitaxial/quasiepitaxial multilayershaving abrupt interfaces. Finally, the mechanical stability ofmolecule-based films can be problematic, especially for large-areaapplications or on flexible backings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B show structural formulae for porphyrinic compounds whichare illustrative examples of compounds of the type which can be used ashole injection components/agents in the preparation of the molecularconductive or hole injection layers and electroluminescent media of thisinvention. In FIG. 1, M is Cu, Zn, SiCl₂, or 2H; Q is N or C(X), where Xis a substituted or unsubstituted alkyl or aryl group; and R is H,trichlorosilyl, trialkoxysilyl, or a moiety having 1 to 6 carbon atomswhich can include trichlorosilyl or trialkoxysilyl groups, substitutedon the C₁-C₄, C₈-C₁₁, C₁₅-C₁₈ and/or C₂₂-C₂₅ positions. In FIG. B, M isCu, Zn, SiCl₂, or 2H; Q is N or C(X), where X is a substituted orunsubstituted alkyl or aryl group; and T₁/T₂ is H, trichlorosilyl,trialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can includetrichlorosilyl or trialkoxysilyl groups.

FIGS. 2A-2C show structural formulae for arylamine compounds which areillustrative examples of compounds of the type which can be used as holetransport compounds/agents in the preparation of the molecularconductive or hole transport layers and electroluminescent media of thisinvention. In FIG. 2A, R₂, R₃ and/or R₄ can be H, trihalosilyl,trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6carbon atoms which can include dialkyldichlorosilyl,dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groupssubstituted anywhere on the aryl positions. In FIG. 2B, Q₁ and Q₂ can besubstituted or unsubstituted tertiary aryl amines, such as thosedescribed with FIG. 2A; and G is a linking group to include but notlimited to an alkyl, aryl, cylcohexyl or heteroatom group. In FIG. 2C,Ar is an arylene group; n is the number of arylene groups from 1-4; andR₅, R₆, R₇, and/or R₈ can be H, trihalosilyl, trialkoxysilyl,dihalosilyl, dialkoxysilyl or a moiety having 1 to 6 carbon atoms whichcan include dialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilylor trialkoxysilyl groups substituted anywhere on the aryl positions.

FIGS. 3A-3C show structural formulae for aryl compounds which areillustrative of examples of compounds of the type which can be used asemissive compounds/agents in the preparation of the molecular conductivelayers and electroluminescent media of this invention. In FIG. 3A, R₉and R₁₀ can be H, trihalosilyl, trialkoxysilyl, dihalosilyl,dialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can includedialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl ortrialkoxysilyl groups substituted anywhere on the aryl positions. InFIG. 3B, M is Al or Ga; and R₁₁-R₁₄ can be H, trihalosilyl,trialkoxysilyl, dihalosilyl, dialkoxysilyl, or a moiety having 1 to 6carbon atoms which can include dialkyldichlorosilyl,dialkyldialkoxysilyl, trichlorosilyl or trialkoxysilyl groupssubstituted anywhere on the aryl positions. In FIG. 3C, Ar is arylene;and R₁₅-R₁₈ can be H, trihalosilyl, trialkoxysilyl, dihalosilyl,dialkoxysilyl, or a moiety having 1 to 6 carbon atoms which can includedialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl ortrialkoxysilyl groups substituted anywhere on the aryl positions.

FIGS. 4A-4C show structural formulae for heterocyclic compounds whichare illustrative examples of compounds of the type which can be used aselectron transport components/agents in the preparation of the molecularconductive or electron transport layers and in electroluminescent mediaof this invention. In FIGS. 4A-4C, X is O or S; and R₁₉-R₂₄ can be arylgroups substituted with the following substituents anywhere on the arylring: trihalosilyl, trialkoxysilyl, dihalosilyl, dialkoxysilyl, or amoiety having 1 to 6 carbon atoms which can containdialkyldichlorosilyl, dialkyldialkoxysilyl, trichlorosilyl ortrialkoxysilyl groups.

FIGS. 5A and 5B (ITO is indium-tin-oxide; HTL is hole transport layerand ETL is electron transport layer) show, schematically and in astep-wise manner by way of illustrating the present invention, use ofthe components/agents of Examples 1-5 and FIGS. 1-4 in the self-assemblyand preparation of an organic light-emitting diode device. Inparticular, the molecular representation FIG. 5A illustrates thehydrolysis of an assembled silicon/silane component/agent to provide anSi—OH functionality reactive toward a silicon/silane moiety of anothercomponent, agent or conductive layer. The block and molecularrepresentations of FIG. 5B illustrate a completed assembly.

FIG. 6 shows an alternative synthetic sequence enroute to severalarylamine components/agents, also in accordance with the presentinvention.

FIG. 7 shows, schematically and by way of illustrating an alternativeembodiment of the present invention, use of the components/agents ofFIG. 6 in the preparation of another representative electroluminescentdevice.

FIG. 8 graphically correlates x-ray reflectivity measurements of filmthickness with the number of capping layers applied to a substrate. Ascalculated from the slope of the line (y=8.3184x), each layer is about7.84 Å in dimensional thickness.

FIG. 9 graphically shows cyclic voltametry measurements, using 10⁻³Mferrocene in acetonitrile, taken after successive layer (c-e) depositionand as compared to a bare ITO electrode (a). Even one capping layer (b),in accordance with this invention, effectively blocks the electrodesurface. Complete blocking is observed after deposition of three or fourlayers. The sweep rate was 100 mV/sec, and the electrode area was about0.7 cm².

FIGS. 10A-C graphically illustrate various utilities and/or performancecharacteristics (current density, quantum efficiency and forward lightoutput, respectively, versus voltage) achievable through use of thepresent invention, as a function of the number of capping layers on anelectrode surface. 0 layers, bare ITO (< >), 1 layer, 8 Å (□), 2 layers,17 Å (●), 3 layers, 25 Å (▴) and 4 layers, 33 Å (∇). Reference is madeto example 10.

FIG. 11A shows molecular structures of hole adhesion/injection molecularcomponents: TAA (shown after crosslinking), TPD-Si₂ (shown aftercrosslinking), and prior art copper phthalocyanine, Cu(Pc). FIG. 11Billustrates one possible scheme for the synthesis of a preferred TPD-Si₂adhesion/injection interlayer molecular precursor. Reference is alsomade to the procedures described in Example 2.

FIGS. 12A-D provide optical microscopic images of vapor-deposited TPDfilm (100 nm) morphology after annealing at 80° C. for 1.0 h on ITOsubstrates coated with a cured 40 nm thick TPD-Si₂ film (12A) and onbare ITO (12B); polarized optical image of TPD film (100 nm) morphologybefore (12C) and after (12D) annealing the bilayer structure:ITO/CuPc(10 nm)/TPD (100 nm) at 80° C. for 0.50 h.

FIGS. 13A-C show, in turn, (A) light output, (B) external quantumefficiency, and (C) current-voltage characteristics as a function ofoperating voltage for OLED devices having the structure:ITO/(adhesion/injection/molecular component interlayer)/TPD holetransport layer (50 nm)/Alq (60 nm)/Al, where the injection/adhesioncomponent interlayer is prior art Cu(Pc) (10 nm), TAA (15 nm), andTPD-Si₂ (40 nm).

FIG. 14 compares injection characteristics of hole-only devices havingthe structure ITO/molecular component interlayer/TPD (250 nm)/Au(5nm)/Al (180 nm) for various anode functionalization layers.

FIG. 15 graphically illustrates by comparison the effect of thermalstressing (90° C. under vacuum) on device characteristics ofITO/(injection/adhesion interlayer)/TPD(50 nm)/Alq(60 nm)/Al (100 nm)devices, where the molecular component interlayer is TPD-Si₂ (40 nm),TAA (15 nm), and prior art CuPc (10 nm).

SUMMARY OF THE INVENTION

In light of the foregoing, it is an object of the present invention toprovide electroluminescent articles and/or devices and method(s) fortheir production and/or assembly, thereby overcoming variousdeficiencies and shortcomings of the prior art, including those outlinedabove. It will be understood by those skilled in the art that one ormore aspects of this invention can meet certain objectives, while one ormore other aspects can meet certain other objectives. Each objective maynot apply equally, in all its respects, to every aspect of thisinvention. As such, the following objects can be viewed the alternativewith respect to any one aspect of this invention.

It is an object of the present invention to provide control over thethickness dimension of a luminescent medium and/or the conductive layersof such a medium, to control the wavelength of light emitted from anyelectroluminescent device and enhance the efficiency of such emission.

It can be another object of the present invention to provide molecularcomponents for the construction and/or modification of anelectroluminescent medium and/or the conductive layers thereof, whichwill allow lower driving and/or turn-on voltages than are availablethrough use of conventional materials.

It can also be an object of the present invention to provide componentmolecules which can be used effectively in liquid media without resortto high vacuum or MBE growth equipment.

It can also be an object of the present invention to provide conformalconductive layers and the molecular components thereof which allows forthe smooth, uniform deposition on an electrode, substrate surface and/orpreviously-deposited layers.

It can also be an object of this invention to provide anelectroluminescent medium having a hybrid structure and where one ormore of the layers is applied by a spin-coat or vapor depositiontechnique to one or more self-assembled conductive layers.

Other objects, features and advantages of the present invention will beapparent from this summary of the invention and its descriptions ofvarious preferred embodiments, and will be readily apparent to thoseskilled in the art having knowledge of various electroluminescentdevices and assembly/production techniques. Such objects, features,benefits and advantages will be apparent from the above as taken intoconjunction with the accompanying examples, data, figures and allreasonable inferences to be drawn therefrom, alone or with considerationof the references incorporated herein.

This invention describes, in part, a new route to the fabrication oflight-emitting organic multilayer heterojunction devices, useful forboth large and small, multicolored display applications. As describedmore fully below, electron and hole transporting layers, as well as theemissive layer, as well as any other additional layers, are applied,developed and/or modified by molecular self-assembly techniques. Assuch, the invention can provide precise control over the thickness of aluminescent medium or the conductive layers which make up such a medium,as well as provide maximum light generation efficiency. Use of thepresent invention provides strong covalent bonds between the constituentmolecular components, such that the mechanical, thermal, chemical and/orphotochemical stability of such media and/or conductive layers, as canbe used with an electroluminescent device, are enhanced. The use of suchcomponents also promotes conformal surface coverage to prevent cracksand pinhole deformities.

More specifically, the siloxane self-assembly techniques describedherein allow for the construction of molecule-based electroluminescentmedia and devices. As described more fully below, various molecularcomponents can be utilized to control the thickness dimension of theluminescent media and/or conductive layers. Nanometer dimensions can beobtained, with self-sealing, conformal coverage. The resulting covalent,hydrophobic siloxane network imparts considerable mechanical strength,as well as enhancing the resistance of such media and/or devices todielectric breakdown, moisture intrusion, and other degradativeprocesses.

In part, the present invention is an electroluminescent article ordevice which includes (1) an anode, (2) a plurality of molecularconductive layers where one of the layers is coupled to the anode withsilicon-oxygen bonds and each of the layers is coupled one to anotherwith silicon-oxygen bonds, and (3) a cathode in the electrical contactwith the conductive layers. More generally and within the scope of thisinvention, an anode is separated from a cathode by an organicluminescent medium. The anode and the cathode are connected to anexternal power source by conductors. The power source can be acontinuous direct, alternating or an intermittent current voltagesource. A convenient conventional power source, including any desiredswitching circuitry, which is capable of positively biasing the anodewith respect to the cathode, can be employed. Either the anode orcathode can be at ground potential.

The conductive layers can include but are limited to a hole transportlayer, a hole injection layer, an electron transport layer and anemissive layer. Under forward biasing conditions, the anode is at ahigher potential than the cathode, and the anode injects holes (positivecharge carriers) into the conductive layers and/or luminescent mediumwhile the cathode injects electrons therein. The portion of thelayers/medium adjacent to the anode forms a hole injecting and/ortransporting zone while the portion of the layers/medium adjacent to thecathode forms an electron injecting and/or transporting zone. Theinjected holes and electrons each migrate toward the oppositely chargedelectrode, resulting in hole-electron interaction within the organicluminescent medium of conductive layers. A migrating electron drops fromits conduction potential to a valence band in filling a hole to releaseenergy as light. In such a manner, the organic luminescent layers/mediumbetween the electrodes performs as a luminescent zone receiving mobilecharge carriers from each electrode. Depending upon the construction ofthe article/device, the released light can be emitted from theluminescent conductive layers/medium through one or more of edgesseparating the electrodes, through the anode, through the cathode, orthrough any combination thereof. See, U.S. Pat. No. 5,409,783 and, inparticular cols. 4-6 and FIG. 1 thereof, which is incorporated herein byreference in its entirety. As would be understood by those skilled inthe art, reverse biasing of the electrodes will reverse the direction ofmobile charge migration, interrupt charge injection, and terminate lightemission. Consistent with the prior art, the present inventioncontemplates a forward biasing DC power source and reliance on externalcurrent interruption or modulation to regulate light emission.

As demonstrated and explained below, it is possible to maintain acurrent density compatible with efficient light emission while employinga relatively low voltage across the electrodes by limiting the totalthickness of the organic luminescent medium to nanometer dimensions. Atthe molecular dimensions possible through use of this invention, anapplied voltage of less than about 10 volts is sufficient for efficientlight emission. As discussed more thoroughly herein, the thickness ofthe organic luminescent conductive layers/medium can be designed tocontrol and/or determine the wavelength of emitted light, as well asreduce the applied voltage and/or increase in the field potential.

Given the nanometer dimensions of the organic luminescent layers/medium,light is usually emitted through one of the two electrodes. Theelectrode can be formed as a translucent or transparent coating, eitheron the organic layer/medium or on a separate translucent or transparentsupport. The layer/medium thickness is constructed to balance lighttransmission (or extinction) and electrical conductance (or resistance).Other considerations relating to the design, construction and/orstructure of such articles or devices are as provided in the abovereferenced U.S. Pat. No. 5,409,783, such considerations as would bemodified in accordance with the molecular conductive layers and assemblymethods of the present invention.

In preferred embodiments, the conductive layers have molecularcomponents, and each molecular component has at least two siliconmoieties. In highly preferred embodiments, each silicon moiety is ahalogenated or alkoxylated silane and silicon-oxygen bonds areobtainable from the condensation of the silane moieties with hydroxyfunctionalities. In preferred embodiments, the present invention employsan anode with a substrate having a hydroxylated surface portion. Thesurface portion is transparent to near-IR and visible wavelengths oflight. In such highly preferred embodiments the hydroxylated surfaceportions include SiO₂, In₂.xSnO₂, Ge and Si, among other such materials.

In conjunction with anodes and the hydroxylated surface portionsthereof, the conductive layers include molecular components, and eachmolecular component has at least two silicon moieties. As discussedabove, in such embodiments, each silicon moiety is a halogenated oralkoxylated silane, and silicon-oxygen bonds are obtainable from thecondensation of the silane moieties with hydroxy functionalities whichcan be on a surface portion of an anode. Consistent with such preferredembodiments, a cathode is in electrical contact with the conductivelayers. In highly preferred embodiments, the cathode is vapor depositedon the conductive layers, and constructed of a material including Al,Mg, Ag, Au, In, Ca and alloys thereof.

In part, the present invention is a method of producing a light-emittingdiode having enhanced stability and light generation efficiency. Themethod includes (1) providing an anode with a hydroxylated surface; (2)coupling the surface to a hole transport layer having a plurality ofmolecular components, with each component having at least two siliconmoieties reactive with the surface, with coupling of one of the siliconmoieties to form silicon-oxygen bonds between the surface and the holetransport layer; (3) coupling the hole transport layer to an electrontransport layer, the electron transport layer having a plurality ofmolecular components with each of the components having at least twosilicon moieties reactive with the hole transport layer, with thecoupling of one of the silicon moieties to form silicon-oxygen bondsbetween the hole and electron transport layers; and (4) contacting theelectron transport layer with a cathode material.

In preferred embodiments of this method, the hole transport layerincludes a hole injecting zone of molecular components and a holetransporting zone of molecular components. Likewise, in preferredembodiments, each silicon moiety is a halogenated or alkoxylated silanesuch that, with respect to this embodiment, coupling the hole transportlayer to the electron transport layer further includes hydrolyzing thehalogenated or alkoxylated silane. Likewise, with respect to ahalogenated or alkoxylated silane embodiment, contacting the electrontransport layer with the cathode further includes hydrolyzing thesilane.

In part, the present invention is a method of controlling the wavelengthof light emitted from an electroluminescent device. The inventive methodincludes (1) providing in sequence a hole transport layer, an emissivelayer and an electron transport layer to form a medium of organicluminescent layers; and (2) modifying the thickness dimension of atleast one of the layers, each of the layers including molecularcomponents corresponding to the layer and having at least two siliconmoieties reactive to a hydroxy functionality and the layers coupled oneto another by Si—O bonds, the modification by reaction of thecorresponding molecular components one to another to form Si—O bondsbetween the molecular components, and the modification in sequence ofthe provision of the layers.

In preferred embodiments of this inventive method, at least one siliconmoiety is unreacted after reaction with a hydroxy functionality. Inhighly preferred embodiments, modification then includes hydrolyzing theunreacted silicon moiety of one of the molecular components to form ahydroxysilyl functionality and condensing the hydroxysilyl functionalitywith a silicon moiety of another molecular component to form a siloxanebond sequence between the molecular components.

In highly preferred embodiments, the silicon moieties are halogenated oralkoxylated silane moieties. Such embodiments include modifying thethickness dimension by hydrolyzing the unreacted silane moiety of one ofthe molecular components to form a hydroxysilyl functionality andcondensing the hydroxysilyl functionality with a silane moiety ofanother molecular component to form a siloxane bond sequence between themolecular components.

While the organic luminescent conductive layers/medium of this inventioncan be described as having a single organic hole injecting ortransporting layer and a single electron injecting or transportinglayer, modification of each of these layers with respect to dimensionalthickness or into multiple layers, as more specifically described below,can result in further refinement or enhancement of device performance byway of the light emitted therefrom. When multiple electron injecting andtransporting layers are present, the layer receiving holes is the layerin which hole-electron interaction occurs, thereby forming theluminescent or emissive layer of the device.

The articles/devices of this invention can emit light through either thecathode or the anode. Where emission is through the cathode, the anodeneed not be light transmissive. Transparent anodes can be formed ofselected metal oxides or a combination of metal oxides having a suitablyhigh work function. Preferred metal oxides have a work function ofgreater than 4 electron volts (eV). Suitable anode metal oxides can bechosen from among the high (>4 eV) work function materials. Atransparent anode can also be formed of a transparent metal oxide layeron a support or as a separate foil or sheet.

The devices/articles of this invention can employ a cathode constructedof any metal, including any high or low work function metal, heretoforetaught to be useful for this purpose and as further elaborated in thatportion of the incorporated patent referenced in the precedingparagraph. As mentioned therein, fabrication, performance, and stabilityadvantages can be realized by forming the cathode of a combination of alow work function (<4 eV) metal and at least one other metal. Availablelow work function metal choices for the cathode are listed in cols.19-20 of the aforementioned incorporated patent, by periods of thePeriodic Table of Elements and categorized into 0.5 eV work functiongroups. All work functions provided therein are from Sze, Physics ofSemiconductor Devices, Wiley, N.Y., 1969, p.366.

A second metal can be included in the cathode to increase storage andoperational stability. The second metal can be chosen from among anymetal other than an alkali metal. The second metal can itself be a lowwork function metal and thus be chosen from the above-referenced listand having a work function of less than 4 eV. To the extent that thesecond metal exhibits a low work function it can, of course, supplementthe first metal in facilitating electron injection.

Alternatively, the second metal can be chosen from any of the variousmetals having a work function greater than 4 eV. These metals includeelements resistant to oxidation and, therefore, those more commonlyfabricated as metallic elements. To the extent the second metal remainsinvariant in the article or device, it can contribute to the stability.Available higher work function (4 eV or greater) metal choices for thecathode are listed in lines 50-69 of col. 20 and lines 1-15 of col. 21of the aforementioned incorporated patent, by periods of the PeriodicTable of Elements and categorized into 0.5 eV work function groups.

As described more fully in U.S. Pat. No. 5,156,918 which is incorporatedherein by reference in its entirety, the electrodes and/or substrates ofthis invention have, preferably, a surface with polar reactive groups,such as a hydroxyl (—OH) group. Materials suitable for use with or aselectrodes and/or substrates for anchoring the conductive layers andluminescent media of this invention should conform to the followingrequirements: any solid material exposing a high energy (polar) surfaceto which layer-forming molecules can bind. These may include: metals,metal oxides such as SiO₂, TiO₂, MgO, and Al₂O₃ (sapphire),semiconductors, glasses, silica, quartz, salts, organic and inorganicpolymers, organic and inorganic crystals and the like.

Inorganic oxides (in the form of crystals or thin films) are especiallypreferred because oxides yield satisfactory hydrophilic metal hydroxylgroups on the surface upon proper treatment. These hydroxyl groups reactreadily with a variety of silyl coupling reagents to introduce desiredcoupling functionalities that can in turn facilitate the introduction ofother organic components.

The physical and chemical nature of the anode materials dictatesspecific cleaning procedures to improve the utility of this invention.Alkaline processes (NaOH aq.) are generally used. This process willgenerate a fresh hydroxylated surface layer on the substrates while themetal oxide bond on the surface is cleaved to form vicinal hydroxylgroups. High surface hydroxyl densities on the anode surface can beobtained by sonicating the substrates in an aqueous base bath. Thehydroxyl groups on the surface will anchor and orient any of themolecular components/agents described herein. As described more fullybelow, molecules such as organosilanes with hydrophilic functionalgroups can orient to form the conductive layers.

Other considerations relating to the design, material choice andconstruction of electrodes and/or substrates useful with this inventionare as provided in the above referenced and incorporated U.S. Pat. No.5,409,783 and in particular cols. 21-23 thereof, such considerations aswould be modified by those skilled in the art in accordance with themolecular conductive layers, and assembly methods and objects of thepresent invention.

The conductive layers and/or organic luminescent medium of thedevices/articles of this invention preferably contain at least twoseparate layers, at least one layer for transporting electrons injectedfrom the cathode and at least one layer for transporting holes injectedfrom the anode. As is more specifically taught in U.S. Pat. No.4,720,432, incorporated herein by reference in its entirety, the latteris in turn preferably at least two layers, one in contact with theanode, providing a hole injecting zone and a layer between the holeinjecting zone and the electron transport layer, providing a holetransporting zone. While several preferred embodiments of this inventionare described as employing at least three separate organic layers, itwill be appreciated that either the layer forming the hole injectingzone or the layer forming the hole transporting zone can be omitted andthe remaining layer will perform both functions. However, enhancedinitial and sustained performance levels of the articles or devices ofthis invention can be realized when separate hole injecting and holetransporting layers are used in combination.

Porphyrinic and phthalocyanic compounds of the type described in cols.11-15 of the referenced/incorporated U.S. Pat. No. 5,409,783 can be usedto form the hole injecting zone. In particular, the phthalocyaninestructure shown in column 11 is representative, particularly where X canbe, but is not limited to, an alkyltrichlorosilane,alkyltrialkoxysilane, dialkyldialkoxysilane, or dialkyldichlorosilanefunctionality and where the alkyl and alkoxy groups can contain 1-6carbon atoms or is hydrogen. Preferred porphyrinic compounds arerepresented by the structure shown in col. 14 and where R, T¹ and T² canbe but are not limited to an alkyltrichlorosilane, alkyltrialkoxysilane,dialkyldialkoxysilane, or dialkyldichlorosilane functionality and wherethe alkyl and alkoxy groups contain 1-6 carbon atoms or is hydrogen.(See, also, FIGS. 1A and 1B, herein.) Preferred phthalocyanine- andporphyrin-based hole injection agents include silicon phthalocyaninedichloride and 5,10,15,20-tetraphenyl-21H,23H-porphine silicon (IV)dichloride, respectively.

The hole transporting layer is preferably one which contains at leastone tertiary aromatic amine, examples of which are as described in FIGS.2A-2C and Examples 1 and 2. Other exemplary arylamine core structuresare illustrated in U.S. Pat. No. 3,180,730, which is incorporated hereinby reference in its entirety, where the core structures are modified asdescribed herein. Other suitable triarylamines substituted with a vinylor vinylene radical and/or containing at least one active hydrogencontaining group are disclosed in U.S. Pat. Nos. 5,409,783, 3,567,450and 3,658,520. These patents are incorporated herein by reference intheir entirety and the core structures disclosed are modified asdescribed herein. In particular, with respect to the arylaminesrepresented by structural formulas XXI and XXIII in cols. 15-16 of U.S.Pat. No. 5,409,703, R²⁴, R²⁵, R²⁶, R²⁷, R³⁰, R³¹ and R³² can be analkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane, ordialkyldichlorosilane functionality where the alkyl and alkoxy groupscan contain 1-6 carbon atoms or is hydrogen.

Molecular components of this invention comprising emissive agents and/orthe emissive layer include those described herein in FIGS. 3A-3C andExample 5. Other such components/agents include various metal chelatedoxinoid compounds, including chelates of oxine (also commonly referredto as 8-quinolinol or 8-hydroxyquinoline), such as those represented bystructure III in col. 8 of the referenced and incorporated U.S. Pat. No.5,409,783, and where Z² can be but is not limited to analkyltrichlorosilane, alkyltrialkoxysilane, dialkyldialkoxysilane, ordialkyldichlorosilane functionality and where the alkyl and alkoxygroups can contain 1-6 carbon atoms or is hydrogen. Other such molecularcomponents/emissive agents include the quinolinolato compoundsrepresented in cols. 7-8 of U.S. Pat. No. 5,151,629, also incorporatedherein by reference in its entirety, where a ring substituent can be butis not limited to an alkyltrichlorosilane, alkyltrialkoxysilane,dialkyldialkoxysilane, or dialkyldichlorosilane functionality and wherethe alkyl and alkoxy groups can contain 1-6 carbon atoms or is hydrogen.In a similar fashion, the dimethylidene compounds of U.S. Pat. No.5,130,603, also incorporated herein by reference in its entirety, can beused, as modified in accordance with this invention such that the arylsubstituents can include an alkyltrichlorosilane, alkyltrialkoxysilane,dialkyldialkoxysilane, or dialkyldichlorosilane functionality and wherethe alkyl and alkoxy groups can contain 1-6 carbon atoms or is hydrogen.

Other components which can be used as emissive agents include withoutlimitation anthracene, naphthalene, phenanthrene, pyrene, chrysene,perylene and other fused ring compounds, or as provided in col. 17 ofthe previously referenced and incorporated U.S. Pat. No. 5,409,783, suchcompounds as modified in accordance with this invention and as morefully described above. Modifiable components also include thosedescribed in U.S. Pat. Nos. 3,172,862, 3,173,050 and 3,710,167—all ofwhich are incorporated herein by reference in their entirety.

Molecular components which can be utilized as electron injecting orelectron transport agents and/or in conjunction with an electroninjection or electron transport layer are as described in FIGS. 4A-4Cand Examples 3(a)-(d) and 4. Other such components include oxadiazolecompounds such as those shown in cols. 12-13 of U.S. Pat. No. 5,276,381,also incorporated herein by reference in its entirety, as such compoundswould be modified in accordance with this invention such that the phenylsubstituents thereof each include an alkyltrichlorosilane,alkyltrialkoxysilane, dialkyldialkoxysilane, or dialkyldichlorosilanefunctionality and where the alkyl and alkoxy groups can contain 1-6carbon atoms or is hydrogen. Likewise, such components can be derivedfrom the thiadiazole compounds described in U.S. Pat. No. 5,336,546which is incorporated herein by reference in its entirety.

As described above, inorganic silicon moieties can be used inconjunction with the various molecular components, agents, conductivelayers and/or capping layers. In particular, silane moieties can be usedwith good effect to impart mechanical, thermal, chemical and/orphotochemical stability to the luminescent medium and/or device. Suchmoieties are especially useful in conjunction with the methodologydescribed herein. Degradation is minimized until further syntheticmodification is desired. Hydrolysis of an unreacted silicon/silanemoiety provides an Si—OH functionality reactive with a silicon/silanemoiety of another component, agent and/or conductive layer. Hydrolysisproceeds quickly in quantitative yield, as does a subsequentcondensation reaction with an unreacted silicon/silane moiety of anothercomponent to provide a siloxane bond sequence between components, agentsand/or conductive layers.

In general, the molecular agents/components in FIGS. 1-4 can be preparedwith a lithium or Grignard reagent using synthetic techniques known toone skilled in the art and subsequent reaction with halosilane oralkoxysilane reagents. Alternatively, unsaturated olefinic or acetylenicgroups can be appended from the core structures using known synthetictechniques. Subsequently, halosilane or alkoxysilane functional groupscan be introduced using hydrosilation techniques, also known to oneskilled in the art. Purification is carried out using proceduresappropriate for the specific target molecule.

It has been observed previously that the performance characteristics ofelectroluminescent articles of the type described herein can be enhancedby the incorporation of a layer having a dielectric function between theanode and, for instance, a hole transport layer. Previous studies showthat the vapor deposition of thin layers of LiF onto an anode beforedeposition of the other layer(s) improves performance in the areas ofluminescence and quantum efficiency. However, this technique is limitedin that the deposited LiF films are rough, degrade in air and do notform comformal, pinhole-free coatings.

The present invention is also directed to the application ofself-assembly techniques to form layers which cap an electrode, providedielectric and other functions and/or enhance performance relative tothe prior art. Such capping layers are self-assembled films which areconformal in their coverage, can have dimensions less than one nanometerand can be deposited with a great deal of control over the total layerthickness. Accordingly, the present invention also includes anelectroluminescent article or device which includes (1) an anode, (2) atleast one molecular capping layer coupled to the anode withsilicon-oxygen bonds, with each capping layer coupled one to anotherwith silicon-oxygen bonds, (3) a plurality of molecular conductivelayers, with one of the layers coupled to the capping layer withsilicon-oxygen bonds and each conductive layer coupled one to anotherwith silicon-oxygen bonds, and (4) a cathode in electric contact with aconductive layer. Likewise, and in accordance with this invention, thecapping layer can be deposited on a conductive layer and/or otherwiseintroduced so as to be adjacent to a cathode, to enhance overallperformance.

More generally and within the scope of this invention, the anode isseparated from the cathode by an organic luminescent medium. The anodeand cathode are connected to an external power source by conductors. Thepower source can be a continuous direct, alternating or intermittentcurrent voltage source. A convenient conventional power source,including any desired switching circuitry, which is capable ofpositively biasing the anode with respect to the cathode, can beemployed. Either the anode or cathode can be at ground potential.

In preferred embodiments, each conductive and/or capping layer hasmolecular components, and each molecular component has at least twosilicon moieties. In highly preferred embodiments, each such conductiveand/or capping component is a halogenated or alkoxylated silane, andsilicon-oxygen bonds are obtainable from the condensation of the silanemoieties with hydroxy functionalities. Without limitation, a preferredcapping material is octachlorotrisiloxane. The anode and cathode can bechosen and/or constructed as otherwise described herein.

In part, the present invention is a method of using molecular dimensionto control the forward light output of an electroluminescent device. Theinventive method includes (1) providing an electrode and a molecularlayer thereon, the layer coupled to the electrode with first molecularcomponents having at least two silicon moieties reactive to a hydroxyfunctionality; and (2) modifying the thickness of the layer by reactingthe molecular components with second components to form a siloxane bondsequence between the first and second molecular components, the secondmolecular components having at least two silicon moieties also reactiveto a hydroxy functionality.

In preferred embodiments of this inventive method, at least one siliconmoiety is unreacted after reaction with a hydroxy functionality. Inhighly preferred embodiments, the modification further includeshydrolyzing an unreacted silicon moiety of one of the molecularcomponents to form a hydroxysilyl functionality and condensing thehydroxysilyl functionality with a silicon moiety of a third molecularcomponent to form a siloxane bond sequence between the second and thirdmolecular components. In highly preferred embodiments, the siliconmoieties are halogenated or alkoxylated silane moieties.

In part, the present invention also includes any electroluminescentarticle for generating light upon application of an electrical potentialacross two electrodes. Such an article includes an electrode having asurface portion and a molecular layer coupled and/or capped thereon. Thelayer includes molecular components, and each component has at least twosilicon moieties. The layer is coupled to the electrode withsilicon-oxygen bonds. In preferred embodiments, each silicon moiety is ahalogenated silane, and silicon-oxygen bonds are obtained from acondensation reaction. Likewise, and without limitation, the electrodehas a substrate with a hydroxylated surface portion transparent tonear-IR and visible wavelengths of light. Such a layer can be utilizedto cap the electrode and/or enhance performance as otherwise describedherein. More generally, in such an article or any other describedherein, the luminescent medium can be constructed using either theself-assembly techniques described herein or the materials andtechniques of the prior art.

The electroluminescent devices and related methods of this invention candemonstrate various interlayer/interfacial phenomena through choice oflayer/molecular components and design of the resultingelectroluminescent medium. As a point of reference, a number of cathodeand anode interfacial structures can enhance charge injection, hencedevice performance. For instance, with vapor-deposited, anode/TPDN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine/Alq(tris(quinoxalinato)Al(III))/cathode devices of the prior art, adramatic increase in light output and quantum efficiency occurs whenÅ-scale LiF or CsF layers are interposed between the cathode andelectron transport layer (ETL). Such thin dielectric layers are thoughtto lower the Al work function, thus reducing the effective electroninjection barrier (energy level offset between the Alq LUMO and the AlFermi level).

In contrast, modification of the ITO anode—hole transport layer (HTL)interface is somewhat more controllable, although similar mechanisticuncertainties pertain. Thus, a variety of ITO functionalizationapproaches produce phenomenologically similar effects, although lessdramatic than those observed for the interposition of alkali fluoride atAl cathodes. These approaches include deposition onto ITO of nanoscalelayers of various organic acids, copper phthalocyanine, or thicker(30-100 nm) layers of polyaniline or polythiophene (PEDOT), allresulting in somewhat enhanced luminous performance. Explanations forthese phenomena are diverse, ranging from altering interfacial electricfields, balancing electron/hole injection fluence, confining electronsin the emissive layer, reducing injected charge back-scattering, andmoderating anode Fermi level-HTL HOMO energetic discontinuities. Thisdiversity of proposed mechanisms accurately reflects the complexity ofinteractions at OLED interfaces and, in many cases, the lack ofnecessary microstructural information.

In a departure from the prior art, the present invention can beconsidered in the context of one or more structural relationshipsbetween an OLED anode and/or its associated organic layers. Withoutrestriction to any one theory or mode of operation, moderation of thesurface energy mismatch can be effected at a hydrophilic oxideanode-hydrophobic HTL interface, as demonstrated below using nanoscopicself-assembled silyl group functionalized amine components (see, forexample, FIG. 11A and 1-4 TAA layers; 11 Å/layer). Promotinganode/ITO-HTL physical cohesion significantly enhances luminousperformance and durability. Furthermore, as relates to another aspect ofthis invention, a silyl-group functionalized, crosslinkable amine layerhaving the core amine structure of the HTL component, TPD, (TPD-Si₂,FIG. 11A) significantly improves ITO/TPD/Alq/Al device performance andthermal durability (one metric of device stability) to an extentsurprising, unexpected and unattainable with other anodefunctionalization structures. In contrast thereto, the commonly usedcopper phthalocyanine (Cu(Pc); FIG. 11A) anode functionalization layeractually templates crystallization of overlying TPD films at modesttemperatures (Example 13 and FIG. 12D), consistent with the thermalinstability of many Cu(Pc)-buffered OLED devices of the prior art.

Accordingly, in its broader respects, the present invention contemplatesa method of using an amine molecular component to enhance hole injectionacross the electrode-organic interface of a light emitting diode device.The inventive method includes (1) providing an anode; and (2)incorporating an electroluminescent medium adjacent the anode, themedium including but not limited to a molecular layer, coupled to theanode, of amine molecular components substituted with at least one silylgroup, and thereon a hole transport layer of molecular components havingthe amine structure of the aforementioned molecular layer components.The molecular layer can have at least one of an arylamine component andan arylalkylamine component, including but not limited to thosemonoarylamine, diarylamine and triarylamine components described in theaforementioned and incorporated U.S. Pat. No. 5,409,783, modified and/orsilyl-functionalized as provided herein. Other suitable arylamine and/orarylalkylamine structures are disclosed in U.S. Pat. Nos. 3,180,730,3,567,450 and 3,658,520, each of which is incorporated herein in itsentirety, such structures as can also be modified to providesilyl-functionality in accordance herewith. Likewise, a combination ofsuch silyl-substituted components can be employed with beneficialeffect.

In preferred embodiments of this inventive method, the aforementionedamine molecular layer components are alkylsilyl-substituted compounds ofthe type illustrated in FIGS. 2A and 2C. In highly preferredembodiments, such components include the alkylsilyl-substituted TAA andalkylsilyl substituted TPD compounds prepared as described herein.Regardless, such a molecular layer can be spin-coated on the anodesurface or self-assembled, as described more fully above, to providesilicon-oxygen bonds therewith. A plurality of such molecular layers canbe coupled successively on an anode surface—each layer coupled one toanother with silicon-oxygen bonds—to improve structural stability andenhance device performance. As described herein and with reference toseveral of the following examples, hole injection can be enhanced bychoice of a molecular layer with components having a structuralrelationship with those arylamine or arylalkylamine components of thehole transport layer. In preferred embodiments, such enhancement can beachieved through use of a silyl-functionalized TPD layer in conjunctionwith a TPD hole transport layer.

As such, the present invention also includes an organicelectroluminescent device for generating light upon application of anelectrical potential cross to electrodes. Such a device includes (1) ananode; (2) at least one molecular layer, coupled to the anode, of one ormore of the aforementioned amine molecular components substituted withat least one silyl group; (3) a conductive layer of molecular componentshaving the amine structure; and (4) a cathode in electrical contact withthe anode. A preferred conductive layer includes a hole transport layercomprising components of the prior art incorporated herein by reference,or modified as described above. Preferred molecular layer components arealkylsilyl-substituted compounds of the type illustrated in FIGS. 2A and2C, in particular silyl-functionalized TAA and TPD. In light of theaforementioned structural relationships and associated methodologies, apreferred conductive layer of such a device is a TPD hole transportlayer, such a layer substantially without crystallization upon annealingand/or at device operation temperatures when used in conjunction with amolecular layer of components having the same or a structurally similaramine structure.

As demonstrated herein, hydrophobic amine HTL—hydrophilic anodeintegrity is a factor in OLED performance; poor physical cohesioncontributes to inefficient hole injection and ultimately, devicefailure. Enhanced performance can be achieved through use of molecularlayer structures which maximize interfacial cohesion and chargetransport. With reference to one preferred embodiment, a convenientlyapplied, spincoated silyl (Si) functionalized TPD analogue, TPD-Si₂(FIGS. 11A-B), structurally similar to the overlying HTL, hencewell-suited to stabilizing the interface, undergoes rapid crosslinkingupon spincoating from solution and subsequent thermal curing to form adense, robust siloxane matrix with imbedded TPD hole-transportcomponents. The thickness of these layers (˜40 nm) was determined byspecular X-ray reflectivity on samples deposited via identicaltechniques on single-crystal silicon. The RMS roughness of the TPD-Si₂molecular layer films on ITO substrates of 30 Å RMS roughness is 8-12 Åby contact mode AFM. Crosslinked TPD-Si₂ films exhibit high thermalstability, with only 5% weight loss observed up to 400° C. by TGA,indicating substantial resistance to thermal degradation. Furthermore,cyclic voltammetry of 40 nm TPD-Si₂ films on ITO electrodes indicatesthat they support facile hole transport and are electrochemicallystable.

The densely crosslinked nature of TPD-Si₂ molecular layer films isevident in the relatively large separation of oxidative and reductivepeaks (200 mV), suggesting kinetically hindered oxidation/reductionprocesses with retarded counterion mobility. P. E. Smolenyak, E. J.Osburn, S.-Y. Chen, L.-K. Chau, D. F. O'Brian, N. R. Armstrong, Langmuir1997, 21, 6568. That TPD-Si₂ film coverage on ITO is conformal andlargely pinhole-free is supported by studies using a previouslydescribed ferrocene probe technique. W. Li, Q. Wang, J. Cui, H. Chou, T.J. Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen, N. Pegyhambarian,P. Dutta, A. J. Richter, J. Anderson, P. Lee, N. Armstrong, Adv. Mater.1999, 11, 730. The lack of significant current flow near the formalpotential for ferrocene oxidation at a TPD-Si₂-coated ITO workingelectrode indicates suppression of ferrocene oxidation, consistent withlargely pinhole-free surface coverage. G. Inzelt, ElectroanalyticalChemistry, Vol. 18, Marcel Dekker, New York, 1994, p. 89.

EXAMPLES OF THE INVENTION

The following non-limiting examples and data illustrate various aspectsand features relating to the articles/devices and/or methods of thepresent invention, including the assembly of a luminescent medium havingvarious molecular components/agents and/or conductive layers, as areavailable through the synthetic methodology described herein. Incomparison with the prior art, the present methods and articles/devicesprovide results and data which are surprising, unexpected and contraryto the prior art. While the utility of this invention is illustratedthrough the use of several articles/devices and molecularcomponents/agents/layers which can be used therewith, it will beunderstood by those skilled in the art that comparable results areobtainable with various other articles/devices andcomponents/agents/layers, as are commensurate with the scope of thisinvention.

Example 1

Synthesis of a Silanated Hole Transport Agent [1]. With reference toreaction scheme, above, hole transport components, agents and/or layerscan be prepared, in accordance with this invention and/or for use inconjunction with light-emitting diodes and other similarelectroluminescent devices. Accordingly, 500 mg. (1.0 mmole) oftrisbromophenylamine (Aldrich Chemical Company) was dissolved in 30 mlof dry dimethoxyethane (DME). This solution was cooled to −45° C. and1.2 ml (3.3 mmole) of a 2.5 M solution of n-butyl lithium in hexane wasadded to the reaction mixture. The entire mixture was then slowly warmedto 20° C. After stirring at 20° C. for an additional hour, the solventwas removed in vacuo. The resulting white precipitate was washed (3×20ml) with dry pentane and redissolved in 30 ml dry DME. This solution wassubsequently poured into 10 ml (87 mmole) of silicon tetrachloride at arate of 1 ml/min. The entire reaction mixture was then refluxed for twohours. The resulting supernatent was separated from the precipitate, andthe solvent again removed in vacuo yielding a green-brown residue. Awhite solid was obtained from this residue upon sublimation at 10⁻⁶torr. Characterization: ¹H NMR (600 MHz, C₆D₆, 20° C.): δ 7.07 (d, 6H,Ar—H); δ 7.05 (d, 6H, Ar—H); EI-MS (m/z): 645 (M+).

Example 2

With reference to FIGS. 2A-2C and the representative arylamines providedtherein, other hole transport agents and/or layers of this invention canbe obtained by straightforward application of the silanation proceduredescribed above in Example 1, with routine synthetic modification(s) andoptimization of reaction conditions as would be well-known to thoseskilled in the art and as required by the particular arylamine.Likewise, preliminary halogenation/bromination can be effected usingknown synthetic procedures. Alternatively, the arylamines of FIGS. 2A-2Cand other suitable substrates can be prepared using other availablesynthetic procedures to provide multiple silane reaction centers for usewith the self-assembly methods and light-emitting diodes of thisinvention. Core molecular substrates of the type from which thearylamines of FIGS. 2A-2C can be prepared are described by Strukelji etal. in Science, 267, 1969 (1995), which is incorporated herein byreference in its entirety.

Example 3

Synthesis of a Silanated Electron Transport Agent. With reference toExamples 3(a)-(d) and corresponding reaction schemes, below, electrontransport agents and/or layers can be prepared, in accordance with thisinvention and/or for use in conjunction with light-emitting diodes andother similar electroluminescent devices.

Example 3a

Synthesis of 4′-Bromo-2-(4-bromobenzoyl)acetophenone [2]. In a 1-literthree neck round bottom flask, 43 g (0.2 mol) methyl 4-bromobenzoic acidand 17.6 g (0.4 mol) sodium hydride were dissolved in 200 ml driedbenzene and heated to 60° C. Next, 39.8 g (0.2 mol) 4-bromoacetophenonein 100 ml dry benzene was slowly added through a dropping funnel, and 1ml methanol was added to the flask to initiate the reaction. After themixture was refluxed overnight, the reaction was quenched by addingmethanol and pouring it into ice water. The pH of the mixture wasbrought down to 7.0 using 5 N sulfuric acid. A solid was collected,washed with water, and recrystallized from benzene to give a lightyellow product. Characterization. Yield: 30.3 g (40%). ¹H NMR (300 MHz,CDCl₃, 20° C., δ): 7.84 (d, 4H, ArH); 7.62 (d, 4H, ArH); 6.77 (s, 2H,CH₂). EI-MS: 382(M+), 301, 225, 183, 157.

Example 3b

Synthesis of 3,5-Bis(4-bromophenyl)isoxazole [3]. In a 250 ml roundbottom flask, 4 g (10.4 mmol) of [2] was dissolved in 100 ml dry dioxaneand heated to reflux, then 3.0 g (43.2 mmol) hydroxylamine hydrogenchloride in 10 ml water and 5 ml (25 mmol) 5 N NaOH was then droppedinto the refluxing mixture. After 12 hours, the reaction mixture wascooled down to room temperature, and the solvent was removed in vacuo.The product was recrystallized from ethanol. Characterization. Yield:3.41 g (85%). M.P. 218.5-219.5° C. ¹H NMR (300 MHz, CDCl₃, 20° C., δ):7.78 (d, 2H, ArH), 7.74 (d, 2H, Ar′H), 7.66 (d, 2H, ArH), 7.6 (d, 2H,Ar′H), 6.82 (s, 1H, isoxazole proton). EI-MS: 379(M+), 224, 183, 155.

Example 3c

Synthesis of 3,5-Bis(4-allylphenyl)isoxazole [4]. In a 250 ml three-neckround bottom flask, 3.77 g (10 mmol) of [3], 460 mg. (0.4 mmol)tetrakis-(triphenylphosphine) palladium, and 7.28 g (22 mmol)tributylallyltin were dissolved in 100 ml. dried toluene and degassedwith nitrogen for 30 min. The mixture was heated to 100° C. for 10 h,then cooled down to room temperature. Next, 50 ml. of a saturatedaqueous ammonium fluoride solution was subsequently added to themixture. The mixture was extracted with ether, and the combined organiclayer was washed by water, then brine, and finally dried over sodiumsulfate. The solvent was removed in vacuo. The residue was purified bycolumn chromatography. (first, 100% hexanes, then chloroform:hexanes[80:20]). Characterization. Yield: 1.55 g (57%). ¹H NMR (300 MHz, CDCl₃,20° C., δ): 7.78 (d, 2H, ArH), 7.74 (d, 2H, Ar′H), 7.34 (d, 2H, ArH),7.30 (d, 2H, Ar′H), 6.78 (s, 1H, isoxazole proton), 5.96 (m, 2H, alkeneH), 5.14 (d, 4H, terminal alkene H), 3.44 (d, 4H, methylene group).EI-MS: 299(M+), 258, 217.

Example 3d

Synthesis of 3,5-Bis(4-(N-trichlorosilyl)propylphenyl)isoxazole [5]. To2 ml of THF was added 5 mg of [4], 3.4 μl of HSiCl₃ and 0.8 mg. ofH₂PtCl₆ were added to 2 ml of THF. The reaction was heated at 50° C. for14 h. The solvent was then removed in vacuo. A white solid was obtainedfrom this residue upon sublimation at 10⁻⁶ torr. Characterization. ¹HNMR (300 MHz, d⁸-THF, 20° C., δ): 7.72 (d, 2H, ArH), 7.68 (d, 2H, Ar′H),7.36 (d, 2H, ArH), 7.32 (d, 2H, Ar′H), 6.30 (s, 1H, isoxazole); 2.52 (t,2H, CH); 1.55 (m, 4H, CH₂); 0.85 (t, 6H, CH₃).

Example 4

With reference to FIGS. 4A-4C and the representative heterocyclesprovided therein, other electron transport agents and/or layers of thisinvention can be obtained by straight-forward application of thesilanation procedure described above in Example 3, with routinesynthetic modification(s) and optimization of reaction conditions aswould be well-known to those skilled in the art and as required by theparticular heterocyclic substrate. Preliminary halogenation/brominationcan be effected using known synthetic procedures or through choice ofstarting materials enroute to a given heterocycle. Alternatively, theheterocycles of FIGS. 4A-4C and other suitable substrates can beprepared using other available synthetic procedures to provide multiplesilane reaction centers for use with the self-assembly methods andlight-emitting diodes of this invention. Core molecular substrates ofthe type from which the heterocycles of FIGS. 4A-4C can be prepared arealso described by Strukelji et al. in Science, 267, 1969 (1995).

Example 5

With reference to FIGS. 3A-3C and the representative chromophoresprovided therein, emissive agents and/or layers, in accordance with thisinvention, can be obtained by appropriate choice of starting materialsand using halogenation and silanation procedures of the type describedin Examples 1-4, above. Alternatively, other chromophores can besilanated using other available synthetic procedures to provide multiplesilane reaction centers for use with the self-assembly methods andlight-emitting diodes of this invention. Regardless, in accordance withthis invention, such emissive agents or chromophores can be used foremission of light at wavelengths heretofore unpractical or unavailable.Likewise, the present invention allows for the use of multiple agents orchromophores and construction of an emissive layer or layers such that acombination of wavelengths and/or white light can be emitted.

Example 6

Examples 6(a)-6(c) together with FIG. 6 illustrate the preparation ofother molecular components which can be used in accordance with thisinvention.

Example 6a

Synthesis of Tertiary Arylamine [6]. Together, 14.46 g (20 mmole) oftris(4-bromophenyl)amine and 500 ml of dry diethyl ether were stirred at−78° C. under a nitrogen atmosphere. Next, 112.5 ml of a 1.6 Mn-butyllithium solution in hexanes was slowly added to the reactionmixture over 1.5 hours. The reaction was then warmed to −10° C. andstirred for an additional 30 minutes. The reaction was then cooled downagain to −78° C. before the addition of 22 g (0.5 mole) of ethyleneoxide. The mixture was stirred and slowly warmed to room temperatureover 12 hours. Next, 2 ml of a dilute NH₄Cl solution was then added tothe reaction mixture. The solvent was evaporated under vacuum yielding alight green solid. The product was purified using column chromatography.The column was first eluted with chloroform and then with MeOH:CH₂Cl₂(5:95 v/v). The resulting light gray solid was recrystallized usingchloroform to give 1.89 g. Yield: 25%. ¹H NMR (δ, 20° C., DMSO): 2.65(t,6H), 3.57 (q,6H), 4.64 (t,3H), 6.45 (d,6H), 7.09 (d,6H). EI-MS: 377(M⁺), 346 (M⁺−31), 315 (M⁺−62). HRMS: 377.2002. calcd; 377.1991. Anal.Calculated for C₂₄H₂₇NO₃; C, 76.36; H, 7.21; N, 3.71. Found: C, 76.55;H, 7.01; N, 3.52.

Example 6b

Synthesis of Tosylated Arylamine [7]. A pyridine solution of tosylchloride (380 mg in 5 ml) was added over 5 minutes to a pyridinesolution of [6] (500 mg in 10 ml, from Example 6a) cooled to 0° C. Themixture was stirred for 12 hours, then quenched with water and extractedwith chloroform. The organic extract was washed with water, 5% sodiumbicarbonate, and dried with magnesium sulfate. After filtration, thechloroform solution was then evaporated to dryness under vacuum andpurified using column chromatography. The column was first eluted withhexane:CHCl₃ (1:2 v/v) yielding [7]. ¹H NMR (300 MHz, δ, 20° C., CDCl₃):2.45 (s,3H), 2.90 (t,6H), 3.02 (t,3H), 3.70 (t,3H), 4.19 (t,6H), 6.92(d,2H), 6.98 (d,4H), 7.00 (d,4H), 711 (d,2H), 7.32 (d,2H), 7.77 (d,2H).

Example 6c

Synthesis of Tosylated Arylamine [8]. Continuing the chromatographicprocedure similar for 2 (from Example 6b)but changing the elutingsolvent to 100% CHCl₃ yielded [8]. ¹H NMR (300 MHz, δ, 20° C., CDCl₃):2.44 (s,6H), 2.91 (t,3H), 3.02 (t,6H), 3.70 (t,6H), 4.19 (t,3H), 6.92(d,4H), 6.98 (d,2H), 7.00 (d,2H), 7.11 (d,4H), 7.32 (d,4H), 7.77 (d,4H).

Example 7

Using the arylamines of Examples 6 and with reference to FIG. 7, anelectroluminescent article/device also in accordance with this inventionis prepared as described, below. It is understood that the arylaminecomponent can undergo another or a series of reactions with asilicon/silane moiety of another molecular component/agent to provide asiloxane bond sequence between components, agents and/or conductivelayers. Similar electroluminescent articles/devices and conductivelayers/media can be prepared utilizing the various other molecularcomponents/agents and/or layers described above, such as in Examples 1-5and FIGS. 1-4, in conjunction with the synthetic modifications of thisinvention and as required to provide the components with the appropriatereactivity and functionality necessary for the assembly method(s)described herein.

Example 7a

This example of the invention shows how slides can be prepared/cleanedprior to use as or with electrode materials. An indium-tin-oxide(ITO)-coated soda lime glass (Delta Technologies) was boiled in a 20%aqueous solution of ethanolamine for 5 minutes, rinsed with copiousamounts of distilled water and dried for 1 hour at 120° C.;alternatively and with equal effect, an ITO-coated soda lime glass(Delta Technologies) was sonicated in 0.5{overscore (M)} KOH for 20minutes, rinsed with copious amounts of distilled water and thenethanol, and dried for 1 hour at 120° C.

Example 7b

Electroluminescent Article Fabrication and Use. The freshly cleanedITO-coated slides were placed in a 1% aqueous solution of3-aminopropyltrimethoxysilane and then agitated for 5 minutes. Thesecoated slides were then rinsed with distilled water and cured for 1 hourat 120° C. The slides were subsequently placed in a 1% toluene solutionof [7] (or [8] from Example 6) and stirred for 18 hours under ambientconditions. Afterwards, the slides were washed with toluene and curedfor 15 minutes at 120° C. AlQ₃ (or GaQ₃; Q=quinoxalate) was vapordeposited on top of the amine-coated slides. Finally, 750-1000 Å ofaluminum was vapor deposited over the metal quinolate layer. Wires wereattached to the Al and ITO layers using silver conducting epoxy(CircuitWorks™), and when a potential (<7V) was applied, red, orange,and/or green light was emitted from the device.

Example 8

One or more capping layers comprising Cl₃SiOSiCl₂OSiCl₃ are successivelydeposited onto clean ITO-coated glass where hydrolysis of the depositedmaterial followed by thermal curing/crosslinking in air at 125° C.yields a thin (˜7.8 Å) layer of material on the ITO surface. X-rayreflectivity measurements indicate that the total film thicknessincreases linearly with repeated layer deposition, as seen in FIG. 8.Other molecular components can be used with similar effect. Suchcomponents include, without limitation, the bifunctional siliconcompounds described in U.S. Pat. No. 5,156,918, at column 7 andelsewhere therein, incorporated by reference herein in its entirety.Other useful components, in accordance with this invention include thosetrifuctional compounds which cross-link upon curing. As would be wellknown to those skilled in the art and made aware of this invention, suchcomponents include those compounds chemically reactive with both theelectrode capped and an adjacent conductive layer.

Example 9

Cyclic voltametry measurements shown in FIG. 9 using aqueousferri/ferrocyanide show that there is considerable blocking of theelectrode after the deposition of just one layer of the self-assembledcapping material specified in Example 8. Other molecular componentsdescribed, above, show similar utility. Almost complete blocking, asmanifested by the absence of pinholes, is observed after application ofthree layers of capping material.

Example 10

Conventional organic electroluminescent devices consisting of TPD (600Å)/Alq (600 Å)/Mg (2000 Å) were vapor-deposited on ITO substratesmodified with the capping material specified in Example 8. FIGS. 10A-Cshow the behavior of these devices with varying thickness of theself-assembled capping material. These results show that such a materialcan be used to modify forward light output and device quantumefficiency. For a device with two capping layers, higher currentdensities and increased forward light output are achieved at lowervoltages, suggesting an optimum thickness of capping material can beused to maximize performance of an electroluminescent article.

Example 11

This example illustrates how a capping material can be introduced toand/or used in the construction of an electroluminescent article.ITO-coated glass substrates were cleaned by sonication in acetone for 1hour followed by sonication in methanol for 1 hour. The dried substrateswere then reactively ion etched in an oxygen plasma for 30 seconds.Cleaned substrates were placed in a reaction vessel and purged withnitrogen. A suitable silane, for instance a 24 mM solution ofoctachlorotrisiloxane in heptane, was added to the reaction vessel in aquantity sufficient to totally immerse the substrates. (Other suchcompounds include those described in Example 8). Substrates were allowedto soak in the solution under nitrogen for 30 minutes. Following removalof the siloxane solution the substrates were washed and sonicated infreshly distilled pentane followed by a second pentane wash undernitrogen. Substrates were then removed from the reaction vessel washedand sonicated in acetone. Substrates were dried in air at 125° C. for 15minutes. This process can be repeated to form a capping layer ofprecisely controlled thickness.

Example 12

The stability of device-type TPD-Si₂ molecular layer/TPD hole transportlayer interfaces under thermal stress (one measure of durability) wasinvestigated by annealing ITO/TPD-Si₂ (40 nm)/TPD (100 nm) bilayers at80° C. for 1.0 h. The optical image of the annealed TPD film shows noevidence of TPD de-wetting/decohesion (FIG. 12A), indicating that theITO-TPD surface energy mismatch is effectively moderated by theinterfacial TPD-Si₂ molecular layer. In contrast, the bare ITO/TPDinterface exhibits catastrophic de-wetting/de-cohesion under identicalthermal cycling (FIG. 12B), visible even under a layer of Alq). Despiteseemingly similar cohesive effects for both TAA and TPD-Si₂ asinterfacial buffer layers, it is reasonable to suggest that theinterfacial cohesion between TPD-Si₂ and TPD is greater, given closerstructural similarity, evidenced by comparing advancing aqueous contactangles: values for bare ITO, silyl-functionalized TAA, silyl (Si₂)functionalized TPD and TPD film surfaces are 0°, 45°, 70°, and 85°respectively, indicating a closer surface energy match at TPD-TPD-Si₂interfaces.

Example 13

Speculation that one role of Cu(Pc) in enhancing OLED performance mightbe via the above adhesion mechanism led to parallel thermal studies. Incontrast to a preferred alkylsilyl-substituted arylamine TPD-Si₂,Cu(Pc)-buffered ITO does not prevent TPD de-cohesion upon heating totemperatures near/above the TPD glass transition temperature (T_(g)).FIG. 12D illustrates the morphology of a 100 nm TPD film on 10 nm Cu(Pc)following heating at 80° C. It is clearly seen that thermal annealinginduces TPD crystallization on the Cu(Pc) film surface (visible evenunder a layer of Alq), yielding star-shaped dendritic crystallites(as-deposited TPD films on Cu(Pc) are smooth and featureless, FIG. 12C).It is likely that such Cu(Pc)-nucleated crystallization occurs duringlocalized heating in operating OLEDs and contributes to observed deviceinstability.

Example 14

Device characteristics employing spincoated TAA, TPD-Si₂, andvapor-deposited Cu(Pc) hole injection layer/adhesion layers in OLEDshaving the structure ITO/interlayer/TPD/Alq/Al are compared in FIG. 13.Versus the bare ITO system, all of the molecular/bufferlayer-incorporated devices exhibit higher light output, enhanced quantumefficiencies, and lower turn-on voltages. Note that a preferred TPD-Si₂component layer affords ˜15,000 cd/m² of maximum light output, which is10-100× greater than the bare ITO-based device. Similar increases inITO/TPD/Alq/Al-type device performance with electrode functionalizationhave only been reported previously for LiF or CsF-modified Al cathodes,via what remains an unresolved mechanism. It is widely accepted thatconventional ITO/TPD/Alq/Al heterostructures are electron-limited due tothe low Alq electron mobility and Alq LUMO-Al Fermi level energeticmismatch, thus raising the question of why TPD-Si₂ anode modificationproduces similar effects. It is suggested that under conditions ofanode-HTL surface energy mismatch and poor anode-HTL cohesion (anunmodified device of prior art), non-ohmic contacts dominate devicebehavior, resulting in significant hole injection barriers typical ofpoor electrode-organic contact.

Example 15

With reference to FIG. 14, TAA and TPD-Si₂ interfacial/molecularcomponent layers are significantly more effective injection structuresthan conventional Cu(Pc) layers. The maximum light output for aCu(Pc)-based device is ˜1500 cd/m² at 25 V, while that of a TAA-baseddevice is 2600 cd/m², and at a much lower bias voltage (16 V). Theexternal quantum efficiency of the Cu(Pc)-based device also falls wellbelow those based on TAA and TPD-Si₂ anode layers: the maximum quantumefficiency for Cu(Pc) is ˜0.3%, in contrast to those of TPD-Si₂ (˜1.2%)and TAA (˜0.9%). The TPD-Si₂-functionalized device is most efficient,producing a maximum light output ˜10× greater than the Cu(Pc)-buffereddevice, and ˜100× greater than the bare ITO-based device. Improvementsobserved and differences between TPD-Si₂ and silyl-functionalized TAAmolecular layers can be explained, without limitation, in relation to:(1) closer aromatic structural similarity of TPD-Si₂ to TPD, producing astronger interlayer affinity and presumably greater π—π interfacialoverlap, and (2) the higher triarylamine: siloxane linker ratio inTPD-Si₂, consistent with more facile hole hopping via densertriarylamine packing.

Example 16

These findings argue that promotion of ITO-TPD interfacialcontact/adhesion leads to more efficient hole injection due to reducedinterfacial contact resistance. This hypothesis was tested by examiningcharacteristics of hole-only devices having the structure ITO/molecularinterlayer/TPD(250 nm)/Au(6 nm)/Al(80 nm), in which electron injectionat the cathode is blocked. Here the Au layer is deposited byrf-sputtering to avoid excessive heating of the TPD underlayer. The holeinjection capacity falls in the order TPD-Si₂≧TAA>Cu(Pc)>bare ITO (FIG.14). Compared to bare ITO, the silyl-functionalized TAA- andTPD-Si₂—modified anodes enhance the hole current density by 10-100× forthe same field strength, with ITO/TPD-Si₂ being most effective. Thus,when contact resistance at the OLED anode side is reduced and holeinjection increased, the greater electric field induced across the Alqlayer enhances electron injection and transport, affording higher lightoutput and comparable or, in the cases where recombination is moreprobable, enhanced quantum efficiency. In contrast, the present andrelated data for Cu(Pc) devices show that the Cu(Pc) significantlysuppresses hole injection. E. W. Forsythe, M. A. Abkowitz, Y. Gao, J.Phys. Chem. B 2000, 104, 3948; S. C. Kim, G. B. Lee, M. Choi, Y. Roh, C.N. Whang, K. Jeong, Appl. Phys. Lett. 2001, 78, 1445. It is believed, inlight of these results, that Cu(Pc) enhances quantum efficiency viabetter balancing hole and electron injection fluences, rather than byfacilitating hole injection or interfacial stability.

Example 17

To examine cohesion and crystallization effects on device durability,thermal stress tests were carried out on devices based on bare ITO,having a 10 nm Cu(Pc) interlayer, and having a 40 nm TPD-Si₂ interlayer.These were subjected to heating at 95° C. for 0.5 h in vacuum andsubsequently examined for changes in luminous response. The irreversibledegradation of the bare ITO and Cu(Pc)-based devices upon heating at 95°C. for 0.5 h (FIG. 15) is reasonably ascribed to TPD de-wetting andCu(Pc)-nucleated TPD crystallization, respectively. Both processes woulddisrupt the multilayer structure, leading to direct hole injection into,and consequent degradation of, the emissive Alq layer, and possibleamplification of pinholes and defects. In contrast, TPD-Si₂-bufferedmolecular layer devices exhibit enhanced performance after heating,which is presumably a consequence of interfacial reconstruction thatpromotes charge injection. These experiments unambiguously demonstratethat covalently interlinked alkylsilyl-substituted compounds such asTPD-Si₂ and TAA, when used as described herein, offer significantimprovements in stabilizing the anode-HTL interface and promoting holeinjection.

The results of this and several preceding examples, demonstrate that aspincoated, hole injecting TPD-Si₂ layer can significantly increasemaximum OLED device luminence (˜100×) and quantum efficiency (˜6×) bypromoting ITO-TPD interfacial cohesion, hence promoting more efficienthole injection. Devices having a TPD-Si₂ anode adhesion layer afford amaximum luminance level of 15,000 cd/m² in absence of dopants or lowwork function cathodes, while exhibiting excellent thermal stability. Inaddition, the same results demonstrate that Cu(Pc) interlayers nucleateTPD crystallization upon heating above the T_(g) of TPD

Example 18

The synthesis of alkylsilyl-functionalized TAA is as was previouslydescribed, both herein and in the literature. W. Li, Q. Wang, J. Cui, H.Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen, N.Pegyhambarian, P. Dutta, A. J. Richter, J. Anderson, P. Lee, N.Armstrong, Adv. Mater. 1999, 11, 730. TPD, Alq, and Cu(Pc) were obtainedfrom Aldrich and purified by gradient sublimation. All other reagentswere used as received unless otherwise indicated. TPD-Si₂ can besynthesized as provided elsewhere, herein (see Example 2) or accordingto FIG. 11B and Examples 19-21, and further characterized by ¹H NMRspectroscopy and elemental analysis.

Example 19

With reference to FIG. 11B enroute to TPD-Si₂, the synthesis of4,4′-bis[(p-bromophenyl)phenylamino)biphenyl (9). To a solution oftris(dibenzyldeneacetone)dipalladium (0.55 g, 0.6 mmol), andbis-(diphenylphosphino)ferrocene; (0.50 g, 0.9 mmol) in toluene (50 mL),was added 1,4-dibromobenzene (18.9 g, 0.0800 mol) at 25° C., and thesolution stirred under N₂ for 10 min. Subsequently, sodium tert-butoxide(4.8 g, 0.050 mol) and N,N′-diphenylbenzidine (6.8 g, 0.020 mol) wereadded, and the reaction mixture stirred at 90° C. for 12 h. The reactionmixture was subsequently cooled to 25° C. and poured into water. Theorganic layer was separated, and the aqueous layer was extracted withtoluene (3×100 mL). The extract was combined with the original organiclayer, and the solvent was removed in vacuo to give the crude product.This was purified by chromatography on silica gel using hexane: ethylenechloride (6:1) as the eluant. A white solid (6.9 g) was obtained in 50%yield. ¹H NMR (CDCl₃): δ 6.99 (d, J=8.8 Hz, 4H), 7.02-7.16 (m, 10H),7.28 (t, J=7.6 Hz, 4H), 7.34 (d, J=8.8 Hz, 4H), 7.45 (d, J=8.4 Hz, 4H).

Example 20

With further reference to FIG. 11B enroute to TPD-Si₂, the synthesis of4,4′-bis[(p-allylphenyl)′phenylamino]biphenyl (10). To a stirring,anhydrous ether solution (10 mL) of 1(1.02 g, 1.58 mmol) under N₂ wasadded dropwise at 25° C. 1.6 mL (3.5 mmol) n-butyl lithium(2.5 M inhexanes), and the mixture stirred for 2 h. CuI (0.76 g, 4.0 mmol) wasthen added, the reaction mixture cooled to 0° C., and allyl bromide(0.60 g, 5.0 mmol) added in one portion. The solution was stirred for 14h, after which time it was quenched with 100 mL saturated aqueous NH₄⁺Cl⁻ solution, followed by extraction with ether (3×100 mL). Thecombined ether extracts were washed with water (2×100 mL) and brine(2×100 mL), and dried over anhydrous Na₂SO₄. Following filtration,solvent was removed in vacuo to yield a yellow oil. Chromatography onsilica gel with hexane: ethylene chloride (4:1) afforded 0.63 g whitesolid. Yield, 70%. ¹H NMR (CDCl₃) δ 3.40 (d, J=10 Hz, 4H), 5.10-5.20 (m,4H), 6.02 (m, 2H), 6.99-7.10 (m, 2H), 7.10-7.20 (m, 16H), 7.28 (t, J=7.6Hz, 4H), 7.46 (d, J=8.8 Hz, 4H). Anal. Calcd for C₄₂H₃₆N₂: C, 88.68, H6.39, N 5.23 Found, C 87.50, H 6.35, N 4.93.

Example 21

With reference to FIG. 11B enroute to TPD-Si₂, the synthesis of4,4′-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (11). To asolution of 2 (0.32 g, 0.55 mol) in 30 mL CH₂Cl₂ at 25° C. was added agrain of H₂PtCl₆.xH₂O, followed by HSiCl₃ (0.73 g, 5.5 mmol). Thereaction solution was warmed to 30° C. and stirred for 4 h. Removal ofthe solvent in vacuum yielded a dark-yellow oil. A mixture of 50 mLpentane and 10 mL toluene was then added. The resulting solid wasfiltered off, and the filtrate was concentrated under vacuum to aviscous, pale-yellow oil. Yield, 98%. ¹H NMR (CDCl₃): δ 1.45 (t, J=7 Hz,4H), 1.90 (t, J=7 Hz, 4H), 2.70 (brs, 4H), 6.80-7.80 (m, 26H). Anal.Calcd for C ₄₂H₃₈Cl₆N₂Si₂: C 60.07, H 4.57; Found, C 60.52, H 4.87.

Example 22

With reference to Examples 19-21, a wide variety of arylalkylaminemolecular components and their silyl-functionalized analogs can beprepared using straight-forward modifications of the synthetictechniques described herein. For instance, with reference to Example 19,diphenylbenzidene can be mono- or dialkylated with the appropriatehaloalkyl reagent to provide the desired arylalkylamine hole transportlayer component. As would also be well known to those skilled in the artmade aware of this invention, the corresponding silyl-functionalizedmolecular layer component can be prepared via mono- or dialkylation withthe appropriate dihaloalkyl reagent followed by subsequent silation,adopting the procedures illustrated in Examples 20 and 21. Accordingly,by way of further example, the alkylated mono- and diarylaminecomponents, discussed above, and their silyl-functionalized analogs canbe prepared to provide the structurally-related molecular and holetransport layers of this invention, and the enhanced performance and/orhole injection resulting therefrom.

Example 23

TPD-Si₂ and TAA Thin Film Deposition and Characterization. Indium tinoxide (ITO) glass sheets with a resistance of 20Ω/□ from Donnelly Corp.were subjected to a standard literature cleaning procedure. TAA andTPD-Si₂-based buffer layers were spincoated onto cleaned ITO surfacesfrom their respective toluene solutions (10 mg/mL) at 2 Krpm, followedby curing in moist air at 110° C. for 15 min. Cyclic voltammetry ofspincoated TPD-Si₂ films on ITO was performed with a BAS 100electrochemical workstation (scan rate, 100 mV/s; Ag wirepseudo-reference electrode, Pt wire counter electrode, supportingelectrolyte, 0.1 M TBAHFP in anhydrous MeCN). For TPD-Si₂ filmcontiguity assessment, 1.0 mM ferrocene in 0.1 M TBAHFP/MeCN was used asthe probe. Thermogravimetric analysis (TGA) was carried out on an SDT2960 DTA-TGA instrument with a scan rate of 10° C./min under N₂. TGAsample preparation involved drop-coating a TPD-Si₂ solution in toluene(10 mM) onto clean glass substrates under ambient conditions. Followingsolvent evaporation, the TPD-Si₂-coated slides were cured at 120° C. for1 h. Upon cooling, the films were detached from the glass substratesusing a razor blade and collected as powders for TGA characterization.

Example 24

ITO/Buffer Layer/TPD Interfacial Stability Studies. TPD de-cohesionanalysis of the interfacial structures ITO/buffer layer/TPD (100 nm)(spincoated TPD-Si₂, spincoated TAA, vapor-deposited Cu(Pc)) werecarried out in the following manner. Following vapor deposition of50-100 nm TPD films onto the respective buffer layer-coated ITOsubstrates, the samples were annealed at 80-100° C. under N₂ for 1.0 h,and the film morphology subsequently imaged by optical microscopy andAFM.

Example 25

OLED Device Fabrication. OLED devices of the structure:ITO/interlayer/TPD(50 nm)/Alq(60 nm)/Al(100 nm) were fabricated usingstandard vacuum deposition procedures (twin evaporators interfaced to a<1 ppm O₂ glove box facility). Deposition rates for organic and metalwere 2-4 Å/sec and 1-2 Å/sec respectively, at 1×10⁻⁶ Torr. The OLEDdevices were characterized inside a sealed aluminum sample containerunder N₂ using instrumentation described elsewhere. J. Cui, Q. Huang, Q.Wang, T. J. Marks, Langmuir 2001, 17, 2051; W. Li, Q. Wang, J. Cui, H.Chou, T. J. Marks, G. E. Jabbour, S. E. Shaheen, B. Kippelen, N.Pegyhambarian, P. Dutta, A. J. Richter, J. Anderson, P. Lee, N.Armstrong, Adv. Mater. 1999, 11, 730.

Example 26

Device Thermal Stability Evaluations. OLED devices were subjected toheating under vacuum at 95° C. for 0.5 h, and were subsequentlyevaluated for I-V and L-V characteristics as described above.

While the principles of this invention have been described in connectionwith specific embodiments, it should be understood clearly that thesedescriptions are added only by way of example and are not intended tolimit, in any way, the scope of this invention. For instance, thepresent invention can be applied more specifically to the constructionof second-order nonlinear optical materials as have been described inU.S. Pat. No. 5,156,918 which is incorporated herein by reference in itsentirety. Likewise, the present invention can be used in conjunctionwith the preparation of optical waveguides. Another advantages andfeatures will become apparent from the claims hereinafter, with thescope of the claims determined by the reasonable equivalents, asunderstood by those skilled in the art.

1. A method of using an amine molecular component to enhance holeinjection across the electrode-organic interface of a light emittingdiode device, said method comprising: providing an anode; andincorporating an electroluminescent medium adjacent said anode, saidmedium comprising an amine molecular layer, coupled to said anode, saidmolecular layer having at the least one of an arylamine molecularcomponent and an arylalkylamine molecular component, each said componentsubstituted with at least one silyl group, and on said molecular layer ahole transport layer of molecular components having the non-silylsubstituted amine structure of said molecular component.
 2. The methodof claim 1 wherein said molecular layer components are selected from thegroup consisting of compounds

wherein R2-R4 are selected from hydrogen and a silyl moiety and at leastone of R2-R4 is a silyl moiety, R5-R8are selected from hydrogen and asilyl moiety and at least one of R5-R8 is a silyl moiety, Ar is anarylene group, and n is a number selected from 1-4.
 3. The method ofclaim 1 wherein said molecular layer components are selected from thegroup consisting of


4. The method of claim 3 wherein said molecular layer is spin-coated onsaid anode.
 5. The method of claim 3 wherein said anode is immersed in asolution of said molecular layer components.
 6. The method of claim 1wherein a plurality of molecular layers are coupled to said anode. 7.The method of claim 1 wherein said hole transport layer isN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine. 8.The method of claim 7 wherein said hole transport layer is spin-coatedon said anode.
 9. An electroluminescent device for generating light uponapplication of an electrical potential across two electrodes, saiddevice comprising: an anode; at least one amine molecular layer, coupledto said anode, said molecular layer having at least one of an arylaminemolecular component and an arylalkylamine molecular component, each saidcomponent substituted with at least one silyl group; a conductive layerof molecular components having the non-silyl substituted amino structureof said amine molecular layer component; and a cathode in electricalcontact with said anode layer.
 10. The device of claim 9 wherein saidamine molecular layer components are selected from the group consistingof compounds

wherein R2-R4 are selected from hydrogen and a silyl moiety and at leastone of R2-R4 is a silyl moiety, R5-R8 are selected from hydrogen and asilyl moiety and at least one of R5-R8 is a silyl moiety, Ar is anarylene group, and n is a number selected from 1-4.
 11. The device ofclaim 9 wherein said amine molecular layer components are selected fromthe group consisting of


12. The device of claim 11 wherein said conductive layer is a holetransport layer ofN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine. 13.The device of claim 9 wherein a plurality of amine molecular layers arecoupled to said anode.
 14. An electroluminescent device for generatinglight upon application of an electrical potential across two electrodes,said device comprising: an anode; at least one molecular layer, coupledto said anode, of arylamine molecular components substituted with atleast two silyl groups; a hole transport layer ofN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine,said hole transport layer substantially without crystallizationsufficient for decohesion of said hole transport layer upon annealing ata temperature at least about the glass transition temperature of saidhole transport layer; and a cathode in electrical contact with saidanode.
 15. The device of claim 14 wherein said molecular layercomponents are selected from the group consisting of


16. The device of claim 14 wherein a plurality of molecular layers arecoupled to said anode.
 17. The device of claim 14 further including anelectron transport layer.
 18. An electroluminescent device forgenerating light upon application of an electrical potential across twoelectrodes, said device comprising; an anode; at least one molecularlayer, coupled to said anode, of silyl-substitutedN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diaminemolecular components; a hole transport layer ofN,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine; anda cathode in electrical contact with said anode.
 19. The device of claim18 further including, an electron transport layer.
 20. The device ofclaim 18 wherein a plurality of molecular layers are coupled to saidanode.